Tank support for the LNG revolution

A modern descendant of the first LNG containment system for ships could become one of the key technologies carrying gas transport and trading into tomorrow. Joe Evangeilsta reviews its evolution. 

When the new Panama Canal opens in late 2015, it will give easy cross-ocean access to much of the current world LNG carrier fleet. Such ease of transport could help the current regional pricing of natural gas and give rise to a consistent worldwide market in which gas is traded like oil. If that happens, a regular spot market for LNG could develop, presenting new opportunity as LNG traders would need carriers that can travel with partially loaded tanks.

Based on the design of traditional bulk liquid cargo spaces, the IHI-SPB is a prefabricated metal tank designed for transporting liquefied gases. It has a particular combination of characteristics that is drawing intense interest from the energy sector among operators consider- ing building floating LNG facilities and from the shipping sector among owners contemplating the emergence of an LNG spot market.

The SPB "Type B" tank was developed in Japan by IHI and received AIP from ABS in 1983. Photo from ABS Surveyor.

Tank support for gas-fueled initiatives

Developed in Japan by Ishikawajima- Harima Heavy Industries (IHI), the next- generation LNG containment system has spent a long time waiting for its day in the sun. It received approval in principal (AIP) from ABS in 1983, made its first appearance in an LNG carrier in 1993 and now is poised to help the natural gas industry grow, evolve and meet the needs of a changing world.

Although the IHI Group merged its shipbuilding division with Universal Shipbuilding to form Japan Marine United (JMU) in early 2013, the contain- ment system retains its original name: the IHI-SPB tank. SPB stands for self-supporting, prismatic IMO Type B independent tank containment system. The word “independent” means the cargo hold is not integral to the ship’s hull. “Prismatic” refers to the tank’s beveled geometry. And “Type B” denotes its classification under the IMO Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, more commonly known as the International Gas Code (IGC).

The principal attractions of the tank are a proven immunity to sloshing problems that allows LNG ships to go to sea partially loaded – even in harsh weather – and a customizable geometry that, among other benefits, results in the kind of flat-deck vessels needed for floating processing plants

While JMU was not the pioneer of this technology, the company applied sophisticated computer analysis capability to redevelop and improve on the original concept to make it work as a Type B tank and incorporated it in the Sanha, the world’s first FPSO for LPG.

A history of LNG containment

The first maritime LNG containment system was a prismatic tank of the Type A variety, installed in a World War II-era cargo ship converted for gas carriage under ABS class in 1958. Renamed Methane Pioneer, the ship carried LNG from Lake Charles, Louisiana, to the Canvey Island terminal in England. The success of that conversion – the vessel remained in service for a decade and finished up doing LPG storage – may have significant repercussions in light of today’s changing gas markets.

Another ancestor of the SPB tank made LNG transport a global business in 1964,when the first purpose-built LNG carriers, ABS-classed Methane Progress and Methane Princess, entered service. These vessels used an update of the prismatic Type A concept developed by the Conch Co. The inner hull was lined with insulation, and the tanks rested on wooden support blocks.

IHI began building prismatic Type A tanks for LPG, ammonia and ethylene carriage in 1960. After making a name in those sectors, the company focused its engineering efforts in 1980 on the challenge of evolving the Conch concept into a freestanding or self-supporting Type B tank. Rigorous studies were made to substantiate the design, including ship motion analysis, FEM (finite element method) analysis of the tank and hull, fine-mesh FEM analysis of local structures, fatigue analysis and crack propagation analysis. Even the insulation system, which is not load-bearing, was subjected to extensive model tests. The tests further demonstrated the suitability of the system to withstand dynamic loads caused by ship motion and thermal cycling and proved its liquid leakage protection for a continuous 15-day period.

Raising the future through the past

SPB technology first entered service in 1993 aboard Polar Eagle and Arctic Sun, a pair of ABS-classed vessels built at the IHI shipyard in Aichi. Although only two vessels were built with the SPB system, their record during 20 years of nonstop service on one of the world’s most severe and challenging runs thoroughly proved the strength and durability of the tank design. The shipbuilder now sees an opportunity for its containment technology to take an important place in emerging LNG markets and applications.

This design eliminates the sloshing problem and has proven capable of handling partial loading under extreme sea conditions. This makes it a viable candidate for floating terminals, where tanks are constantly in a state of partial filling.

Internally, the tank is modeled on conventional bulk liquid cargo holds – a stiffened plate structure subdivided into four spaces by a centerline liquid-tight bulkhead and swash bulkheads. As in a traditional bulk liquids carrier, the bulkheads control the natural frequency of the cargo. By preventing ship motions from creating resonance with the liquid, they eliminate sloshing problems, and the capability for partial loading allows a ship to quickly leave the berth in the event of an emergency.

New future, new ideas

The performance of the two existing SPB-equipped LNG carriers indicates this technology could help the LNG sector evolve and advance into new markets and services.

The main factor that has kept SPB technology out of the LNG building boom of the last decade was not performance, but price. Until 2012, an SPB containment system for an LNG carrier cost about 15% more than a comparable membrane system. Today, according to the manufacturer, the SPB premium is less than 10% more than a membrane system. As orders increase, there is an expectation that production efficiencies will further lower costs. This could well be aided eventually by efficient licensees building tanks in other countries.

Most enquiries for SPB systems to date have come from energy companies considering floating production, storage and terminal facilities for offshore developments, but recently, with Japan looking to increase LNG imports over the coming decade, interest in the system among domestic shipowners has begun to rise.

As SPB tanks are, by nature, custom- built for each ship, they can be tailored to fit any hullform. This raises the possibility of converting existing ships for LNG service, presenting a potential boon to emerging markets needing shuttle tankers and shipowners looking to change the direction of a half-built vessel. While all this may not mean the coming of a future world fleet containing combination carriers with liquefied gas capacity, or parcel tankers hauling LNG as just another hazardous cargo, it does seem to signal interesting times ahead.

EDITOR’S NOTE: A version of this article appeared in the Fall/Winter 2013 issue of Surveyor, a quarterly magazine from ABS.

ABS-classed Sanha, the world's largest LPG FPSO.

Differentiating among tank types

The IGC Code defines membrane tanks as well as three type categories for independent LNG cargo tanks.

Membrane tanks are non-self-supporting tanks which consist of a thin layer (membrane) supported through insulation by the adjacent hull structure. The membrane is designed in such a way that thermal and other expansion or contraction is compensated for without undue stressing of the membrane. This containment system requires a complete secondary barrier capable of containing the cargo for a 15-day period, and typically the membrane tanks do not exceed a 0.25 bar design vapor pressure; however if the hull scantlings are increased accordingly the design vapor pressure may be increased to 0.7 bar. Today, Gaztransport & Technigaz (GTT’s) systems, which are approved by all major classification societies, have a capacity range for existing vessels of 20.000-266.000cu. m. The tanks are also being considered for smaller-sized LNG carriers and LNG barges, as well as fuel tanks on gas-powered vessels.

Type A tanks are designed primarily using recognized standards of classical ship structural analysis and constructed of a plane surface. The code limits this type of tank to a vapor pressure of less than 0.7 bar, and where minimum design temperature is below -10°C, requires a complete secondary barrier capable of containing the cargo for a period of 15 days in the event of a ruptured or leaking tank.

IMO Type B independent tanks are defined as “designed using model tests, refined analytical tools and analysis methods to determine stress levels, fatigue life and crack propagation characteristics.” One of the key characteristics for Type B designation is compliance with the ”leak before failure” concept, under which crack propagation analysis by fracture mechanics techniques must demonstrate that if a crack in the system should develop, its growth will not be rapid enough to allow excessive leakage into the cargo hold. A partial secondary barrier, which can consist of a spray shield and drip pans, is required for independent Type B tanks with minimum design temperatures below -10°C. Prior to the IHI-SPB, all Type B tanks were spherical vessels of the Moss- Rosenberg design.

Type C tanks are spherical or cylindrical pressure vessels, like those typically seen topsides on LPG carriers. Because these tanks can be made to fit into any available space in the ship, the Type C tank is ideal for the fuel tanks in gas-powered vessels. ABS granted AIP for this application in 2011 when JMU developed a concept design for a gas-fueled containership. This is the type indicated for the fuel tanks being fabricated as part of the design for TOTE’s gas-fueled containerships and Waller Marine’s articulated tug-barges for local LNG distribution and supply.

LNG projects on the rise

By Stephen Gordon, Clarkson Research

Rapid development in the LNG sector underscores the significant role technology will continue to play in the natural gas industry and energy markets. LNG shipping has grown significantly over the last 20 years, accounting for 32% of all natural gas trade in 2012 – up from 24% in 1990. Between 1990 and 2012, LNG trade increased by a compound annual growth rate of 7.2% compared to 5.4% per annum for pipeline gas and 2.4% for global gas demand over the same period. Global LNG trade increased from 52 million metric tons (mmt) in 1990 to 222mmt in 2010. By 2013, trade volumes had increased to an estimated 244mmt.

At the end of 2004, only 10 countries were exporting LNG. At the beginning of April 2014, there were 17 countries (with 89 liquefaction trains) that have LNG liquefaction infrastructure. The total production capability of these units is estimated at 293mmt per annum. Qatar remains the largest exporter, with volumes reaching 78mmt in 2013, equivalent to one-third of global exports.

The import side of the LNG business comprised 107 facilities at locations in 30 countries at the beginning of April 2014. And growth in this sector is expected to continue. There are 16 LNG liquefaction plants under construction, with a further 27 projects that have received FID or are at the FEED stage. These developments, along with other potential projects, are expected to support firm trade growth over the long term, despite short-term delays to project startups. It is worth noting that there is significant potential for export growth in the US and Australia.

Global numbers for 2013 indicate Asian nations accounted for three-quarters of global LNG imports, with Japan, South Korea, India, China and Taiwan ranking as the top five LNG import desti- nations. LNG trade routes between countries have multiplied as well, increasing from 45 in 2003 and 93 in 2008 to 168 in 2013.

Changes in the global LNG carrier fleet also reflect an expansion. In 1996, the fleet stood at 90 ships, nearly doubling to 174 by the start of 2005 and rising to 361 by the start of 2011. Today, the fleet stands at 392 vessels.

With the global demand for gas escalating, the LNG industry is poised for continued growth.

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